1. Field of the Invention
The present invention relates to a ceramic heater, an electrochemical element using the ceramic heater, and an oxygen analyzing device having such an electrochemical element. More particularly, the present invention is concerned with such a ceramic heater which is economical to manufacture and highly durable in operation.
2. Discussion of the Prior Art
There is known a ceramic heater which has a heater element including a heat-generating portion and electrical lead portions for supplying power to the heat-generating portion. These heat-generating portion and the electrical lead portions are formed integrally with a ceramic body or substrate. The heater element, i.e., a mass of the heat-generating and lead portions, is formed of a single electrically conductive metal. For example, an inexpensive ceramic heater uses a comparatively inexpensive non-noble or base metal such as tungsten and molybdenum, for the heater element. However, such a ceramic heater suffers from a problem. More specifically, the base metal of the heat-generating portion of the heater element tends to be oxidized during a long period of use of the heater at a high operating temperature in an oxidizing atmosphere such as air. The oxidation may result in disconnection of the heat-generating portion.
In the light of the above drawback, it has been proposed to form the heater element of a noble metal such as platinum and rhodium. While the use of such a noble metal successfully overcomes the electrical disconnection of the heater element, it necessarily increases the cost of manufacture of the ceramic heater, since the noble metal is considerably more expensive that the base metal. This problem of increase in the material cost is serious in view of the recent tendency for the electrical lead portions to have a comparatively large cross sectional area, for lowering the electrical resistance of the lead portions for efficient consumption of electric power by the heat-generating portion to generate heat. Namely, the lead portions require the use of an accordingly large amount of noble metal, which means an increased material cost of the ceramic heater. Further, it is generally difficult to bond a noble metal to a ceramic substrate with a sufficient adhesive or bonding force, and the terminal parts of the electrical lead portions connected to external lead wires or conductors tend to be easily separated from the ceramic substrate.